There has been a image forming apparatus to irradiate a laser beam to a photoconductive drum of an image-carrying body from an exposing unit to form an electrostatic latent image on the surface of the photoconductive drum, make the electrostatic latent image visible by a developer (toner), and transfer the image onto a paper sheet.
In such a image forming apparatus, a photoconductive drum is rotatably provided below the exposing unit. A charging unit, a developing unit, a transferring unit, and a removing unit are disposed one after another, facing the surface of the photoconductive drum.
The charging unit charges the surface of the photoconductive drum with electrostatic charges. A laser beam emitted from the exposing unit is irradiated to the surface of the photoconductive drum to form an electrostatic latent image on the drum. The electrostatic latent image is made to appear as an visible image by a developer (toner) supplied from the developing unit.
The paper sheet is sent between the photoconductive drum and the transferring unit according to the rotation of the photoconductive drum. The transferring unit transfers the visible image (the image by the developer) on the photoconductive drum onto the sent sheet. The removing unit removes the charges of the sheet after passing through the transferring unit.
The charging unit comprises a metal case, and a electrifying wire provided in the metal case. Application of a D.C. voltage between the electrifying wire and the photoconductive drum causes electric discharge between the wire and the photoconductive drum to electrify the surface of the photoconductive drum. Moreover, there is caused electric discharge between the electrifying wire and the metal case. At the discharge, a direct current flows on a path between the electrifying wire and the photoconductive drum, and the total of both the direct currents becomes the operation current on the charging unit.
The removing unit comprises a metal case and a removing wire provided in the metal case. Application of a D.C. voltage between the removing wire and the metal case causes electric discharge between the removing wire and the metal case to remove the charges (that is, the charges caused on the paper sheet by the transferring unit) on the surface of a paper sheet passing on the removing unit through the transferring unit. And, when there is no sheet on the removing unit, or when the resistance of a sheet on the unit is small even in the presence of a sheet, the electric discharge is also caused between the removing wire and the photoconductive drum. At the electric discharge, a direct current flows through a path between the electrifying wire and the metal case, and a direct current is passed through a path between the electrifying wire and the photoconductive drum. The total of both the direct currents becomes an operation current of the removing unit.
Incidentally, the operation current of the removing unit is considerably small by a factor of about {fraction (1/10)}. Moreover, the polarity of the output of the removing unit is the same as that of the output of the charging unit.
Then, it is conceivable to use one power supply circuit in common both for the removing unit and for the charging unit to reduce the number of the parts and the cost.
However, it is necessary to keep the operation current of the charging unit at a predetermined value at any time. A fluctuation in the operation current of the charging unit causes density irregularities of copied image, particularly when copying photographic images with half tone density.
Considering the above circumstances, the object of the invention is to offer a reliable image forming apparatus to use one power supply circuit in common both for an electrifying unit and for a removing unit, and keep the operation current of the charging unit at a predetermined value at any time.
An image forming apparatus of the invention is to form an electrostatic latent image on an image-carrying body, develop the latent image, and transfer the latent image on to a paper sheet, and comprises:
charging unit to electrify the surface of the image-carrying body with electrostatic charges;
an exposing unit to form an electrostatic latent image on the surface of an image-carrying body by irradiation of light on the surface of the image-carrying body after passing through the charging unit;
an developing unit to develop the electrostatic latent image formed by the exposing unit;
a transferring unit to transfer the image developed by the developing unit on the sheet;
a removing unit to remove the sheet after passing through the transferring unit;
a constant-current power supply circuit to supply operation currents to the charging unit and the removing unit, respectively, and keep the total of the operation currents at a predetermined value;
a correction circuit to detect a fluctuation in the operation current of the removing unit, and correct the predetermined value of the constant-current power supply circuit according to the detection result.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.